1. Field of the Invention
This invention relates to magnetoresistive sensors, and more particularly to methods for fabricating integrated magnetoresistive sensors and the resulting sensor structure.
2. Description of the Related Art
Varying magnetic fields have been used in the past as a sensing mechanism for,moving parts, such as rotating elements in an automobile. The rotating element causes a magnetic field to periodically vary, and the rate of variation is sensed as an indication of the rotational speed and/or position.
A simplified system for measuring the rotational speed of a rotating element is shown in FIG. 1. A wheel or gear 2 has a plurality of spaced protruding teeth 4 and rotates below a magnet 6. A magnetoresistor 8 that is provided on its own substrate is positioned in the magnetic field 10 between the magnet and wheel, while amplification and (if desired) digitizing circuits are provided on one or more separate substrates 12 that are electrically connected to the magnetoresistor 8.
The rotating wheel 2 is formed from a magnetic material, and thus attracts the field from magnet 6. The magnetic field at the magnetoresistor 8 is strongest when one of the teeth 4 is located directly below magnet 6, thus minimizing the distance between the magnet and the wheel. As the wheel rotates the field tends to bend along with the movement of the tooth, and thus traverses a greater distance as the tooth moves away from the magnet. These effects produce a reduction in the magnetic field strength at the magnetoresistor 8, which reaches a minimum when the magnet 6 is midway between two teeth 4. The magnetic field strength at the magnetoresistor increases again as the next tooth approaches, reaching a maximum when that tooth is located directly below the magnet. The field strength at the magnetoresistor thus varies periodically as the wheel continues to rotate, causing the resistance of magnetoresistor 2 to fluctuate in a similar fashion. This results in a periodically varying output from the amplifier circuitry 12.
It would be less expensive and reduce the bulk and complexity of the apparatus if the magnetoresistor could be integrated with the output circuitry in a single monolithic structure. However, this has not be practical in the past. Magnetoresistors have commonly been grown in crystalline form, and then cleaved into individual resistor elements that are simply glued to respective substrates. Other magnetoresistor fabrication techniques are known in which the magnetoresistive element is monolithically grown on a substrate. For example, in U.S. Pat. No. 3,898,359 to Nadkarni, a thin film layer of antimony or arsenic is applied over a layer of indium on an insulating substrate that has been coated with chromium or nickel. The antimony or arsenic film is them chemically combined with the indium to form an InSb or InAs magnetoresistor. A series of transverse indium Hall effect shorting strips are formed on the upper surface of the magnetoresistor to short circuit the Hall fields that would otherwise be built up. In another monolithic magnetoresistor construction, an InSb magnetoresistor resistor is epitaxially grown on a GaAs substrate for use in infrared focal plane arrays; Chiang and Bedair, "Growth of InSb and InAs.sub.1-x Sb.sub.x by OM-CVD", J, Electrochem. Society; Solid-State Science and Technology, October 1984, pages 2422-2426. As with U.S. Pat. No. 3,898,359, this article discloses the fabrication of a magnetoresistor by itself, without any associated circuitry on the same substrate.
In fact, although GaAs is particularly suited as a substrate for InSb because it has thermal expansion characteristics that are very close to those of InSb, InSb is not compatible with the annealing of dopants in GaAs. This is because the annealing is typically performed at about 850.degree. C., while InSb cannot withstand temperatures greater than about 450.degree. C. This is a significant limitation in the achievement of a monolithically integrated magnetoresistor and processing circuit, since InSb is the most magnetosensitive material currently known. It would also not be feasible to grow an InSb magnetoresistor on the same substrate with an integrated circuit that has already been fabricated, since InSb is typically grown at about 400.degree. C., while the ohmic contacts of an integrated circuit begin to degrade at temperatures above about 250.degree. C.
Even if the temperature problems could be overcome, there are other serious obstacles to the monolithic integration of a magnetoresistor with associated output circuitry. To grow a magnetoresistor at a desired location on the substrate, the usual approach would be to provide a growth mask over the entire substrate, with a window at the desired location for the magnetoresistor. InSb would then be grown over the mask and onto the substrate through the window, followed by removing the mask and the overlying InSb to leave the magnetoresistor material only in the window area. However, an InSb magnetoresistor is typically about an order of magnitude thicker than a typical pattern mask, and a continuous layer of the InSb over the entire mask area would prevent the solvent from reaching and removing the mask and leaving a clean substrate surface. It would also not be feasible to grow InSb over the entire substrate and then etch away the unwanted areas, since the etchant would also attack the underlying GaAs.
A magnetosensitive device that is monolithically integrated with associated output circuitry is described in Lepkowski et al., "A GaAs Integrated Hall Sensor/Amplifier", IEEE Electron Device Letters, Vol. Edl-7, No 4, April 1986, pages 222-224. This device, however, uses a Hall sensor rather than a magnetoresistor. The Hall sensor is formed from the GaAs substrate material itself, rather than from a separate magnetoresistor material such as InSb, and therefore does not face the temperature and growth obstacles described above. Although they are easier to integrate with output circuitry, Hall sensors are not as magnetically sensitive as magnetoresistors.